Current sensing circuits using so-called current sensing transistors (or “sense FETs”) have been commonly used for years. Such current sensing techniques may be especially useful when measuring the load current of power field effect transistors (power FETs) which are composed of a plurality of transistor cells as illustrated, for example, in U.S. Patent Application Pub. No. 2001/0020732 A1. Such power field effect transistors have a common drain region for all transistors cells composing the power transistor component. The common drain region is connected by one drain electrode arranged on the back-side of a wafer whereas the source region and the respective source electrodes are contacted on the front-side of the wafer and connected in parallel. The source electrode of a one transistor cell (referred to as “sense cell”) may be separately connected to tap a current signal that is representative of the load current flowing through the plurality of transistor cells of the load transistor. Of course a few transistor cells may be connected in parallel to form the sense transistor.
In circuit arrangement including a load transistor/sense transistor pair the source current of the sense current is directly proportional to the source current of the load transistor whereby the factor of proportionality results from the ratio of the current conducting area of the load transistor and the current conducting area of the sense transistor which is (at least approximately) equivalent to the ratio of the number of transistor cells in the load transistor and, respectively, the sense transistor.
The proportionality condition mentioned above is only met when both transistors (load transistor and sense transistor) operate in the same operating point, i.e., when both transistors are supplied with the same gate-source voltage and are exposed to the same drain-source voltage. A number of circuitry is known which may be applied to ensure that both transistors operate in the same operating point. Just to give an example, for common drain MOS technologies an operational amplifier may be used to set the source-voltage of the sense transistor to match the source-voltage of the load transistor. Due to the common drain electrode an equal drain-source-voltage is achieved. Additionally, the gate electrodes of sense transistor and load transistor are connected so as to provide the same gate-source voltage to both transistors.
Although the sense transistor and load transistor operate in the same operating point due to appropriate circuitry, further undesired side effects and interactions between the both transistors may deteriorate the strict proportionality between the respective source currents. For example, the homogeneous drain current density is to be ensured throughout the transistor cells (of both transistors). An inhomogeneous drain current flow may result in internal traverse currents thus distorting the strict proportional relationship between the source current of the sense transistor and the load transistor.
Due to the structure of the transistors (common drain on the back-side of the wafer, separated source contact for the sense transistor cell) n-type transistors (e.g., n-channel MOSFETs) have to be used as high side switches and p-type transistors (e.g., p-channel MOSFETs) for low side switches. In both cases the source electrodes are exposed to the full voltage swing and, as a consequence, appropriate protection circuitry is required in order to protect the gate oxide which is typically not designed to withstand high voltage differences.